In case you mean the number at "ICQ quality", the lower the number the better the quality (and the larger the file size). 0 is near lossless, 50 is "compressed to death". Sane values are between 18 and 25.
Memory hook for understanding the quality value at all quality-based rate control settings: it can be seen as the amount of video information that is removed to compress better. 0=nothing removed, 50 (maximum value)=50% removed.